Automatic frequency control



Aug 2, 1966 J. STARK, JR

AUTOMATIC FREQUENCY CONTROL Filed June 6, 1965 2 Sheets-Sheet l SVI w United AStates Patent C) 3,264,408 AUTOMATIC FREQUENCY CONTROL .lohn Stark, Jr., Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Filed June 6, 1963, Ser. No. 286,075 6 Claims. (Cl. 178-7.3)

The present invention relates to an automatic frequency control circuit. More particularly the present invention relates to a phase detection circuit for automatically controlling the frequency of the horizontal deflection oscillator in a television receiver.

In television receivers, a local horizontal deflection oscillator controls the repetition rate of the horizontal deflection circuit used for reproducing the televised image on the screen of the cathode ray tube. This oscillator must be properly synchronized with the oscillator at the TV transmitter which originally produced the televised image. The transmitted signal accordingly is a composite signal containing both video information and horizontal and vertical synchronizing signals for the purpose of synchronizing the horizontal and vertical defiection oscillators at the television receiver. In this manner accurate reproduction of the televised image may be obtained.

Phase detector circuits have been developed for cornparing the time of occurrence of the horizontal synchonizing signal with the output of the horizontal oscillator in the television receiver. With such circuits, an output voltage is obtained which is a function of the difference in the time of occurrence between the output of the oscillator and the synchronizing signal.

While these phase detector circuits are generally satisfactory for television viewing purposes there is room for improvement. For example, when the television channel to which the receiver is tuned is changed, there may be momentary loss of synchronization before the control circuit starts to operate properly. Also, if there is loss of synchronization between the receiver and the transmitter for any reason, the recovery time of conventional phase detection circuits may not be as rapid as desired.

It is accordingly an object of the present invention to provide a new and improved automatic frequency control cir-cuit which overcomes the above disadvantages.

A second object of the present invention is to provide a new and improved automatic frequency control for the horizontal oscillator of a telvision'receiver.

Another object of the present invention is to provide a phase detector circuit for a television receiver which has improved frequency response.

Still another object of the present invention is to provide a new and improved phase detector for the horizontal deflection oscillator of a television receiver which detector has a faster recovery time than is presently available.

The present invention contemplates an automatic frequency control arrangement including a pair of peak detector circuits; a first source of periodic signals occurring at a first frequency and having an output applied to the pair of peak detectors .arranged in parallel; a second source of periodic signals occurring at substantially the first frequency and having an output applied to the pair of peak detector circuits arranged in series opposition, the pair of peak detector circuits thereby producing a control voltage which is a function of any phase difference between the periodic signals of the first and second sources; and means tending to balance the voltage developed by the detector circuit. Control means are also included responsive to the control voltage and connected to the second source of periodic signals for changing the rate of occurrence of the periodic signals of the second source in a direction to reduce the difference in frequency 3,264,408 Patented August 2, 1966v lCC of the two periodic signals to zero. The balancing means not only tends to balance the detector circuit voltages, but gives the circuit a faster response, reduces ringing, and has other advantages.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation together with additional objects and advantages thereof will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings in which:

FIGURE l is a schematic diagram partially in block form of a circuit incorporating the principles of the present invention;

FIGURES 2 and 3 are graphical representations of the wave shapes occurring at different portions -of the circuit for the purpose of illustrating the advantageous operation thereof.

Referring to the drawings and more particularly to FIGURE l the composite signals transmitted by a television transmitter are received by an antenna 10 and applied to the radio frequency (RF) tuner and the intermediate frequency (IF) amplifier represented by the block 11. The IF signals produced by the intermediate frequency amplifier of block 11 are applied to the video detector and video amplifier represented by the block 12. At this stage of the television'receiver, in accordance with conventional practice, the video information to be displayed on the screen of the cathode ray tube and the synchronizing signals have been demodulated from the RF carrier signal. The output signal from the video amplifier of the block 12 is applied on conductor 13 to the cathode ray tube 14 to be displayed on the screen thereof.

A second output signal obtained from the block 12 is the modulated sound signal which is applied on conductor 16 to the sound channel 17 wherein it is demodulated and reproduced in synchronism with the displayed image, The apparatus represented by the blocks 11, 12 and 1'7 may be constructed in any one 'of several conventional configurations and a complete illustration of these circuits is not provided in order to avoid unnecessarily complicating the drawings.

A third output signal from the block 12 is applied on conductor 18 to the synchronizing signal separator 19, The synchronizing signal separator 19 operates in a conventional manner to separate the horizontal synchronizing signals from the vertical lsynchronizing signals and from the video signals, all of which are contained in the original signals received by the antenna 10. Y

The vertical synchronizing signals are applied from the sync separator 19 on conductor 21 to a vertical deflection *circuit 22. The vertical deflection `circuit 22 produces a sawtooth current which is applied from its output terminals 23 to the terminals V-V of the vertical deiiecting coils 24 of the cathode ray tube 14.

Also produced at the output of the sync separator 19 on a conductor 26 are the horizontal synchronizing signals which are applied through a coupling capacitor 27 to the junction point 28 between diodes 29 and 31 which form a portion of the phase detection circuit. The diode 29 has a load resistor 32 and a compensating capacitor 30 connected in parallel therewith while diode 31 is connected in parallel with a load resistor 33. The junction between the anode of the diode 31 and the resistor 33 is connected to a source of datum potential or ground for the television receiver.

The junction of the anode of the diode 29, the capacitor 30 and the resist-or 32 is identified by the numeral 34. Junction point 34 is connected to one terminal of a frequency response compensating resistor 38, the other terrninal of which is grounded, and to one terminal of a load capacitor 37 and a resistor 39.

They otherV terminal of capacitor 37 is grounded while resistor 39 is connectedV to one terminal of, and forms an integrating network with a capacitor -41` the other .terminal of which 1s grounded.

` The junction between resistor 39 and capacitor 41 is connected tothe control electrode of a horizontalcontrol tube 40. The control electrode orf tube 40,` is connected to ground by the seriesy combination of a .resistorY 42 and a capacitor 43. The cathode of tube 40` isicon-k thereof is connected by means of anode resistors 45 land nected to ground by a bias resistor 44 and the anodefi 46 to the positivevterminal 47 of fa source of operating i potential. is bypassed to ground by capacitor 55. The anodeot tube 40' is connected to ground through biasing resistor 48 and to -the movable tap of a horizontal hold potentiometer 49.

One end of potentiometer 49 is connected to TheV junction 'between resistors 45 and `46 r ground by means of resistor 5]. while the other end of potentiometer 49 is connected to the control electrode of a horizontal oscillator tube 50 by means of a rel Sisto-1' A52.

The cathode of oscillator tube 50 is connectedto ground by the Ituned circuit 53' made up of a variable inductory and a capacitor. The anode of tube 50 is coupled to the control electrode thereofiby means of a tapped inductor 53 and capacitor 54 in series. The anode of tube 50 is also connected to the positive terminal 47 ott the lsource of operating potential by means of anode `resistors 56, 57 and 46 connected in series from the terminal 47 to the tap on tapped inductor 53. The periodic signals from-` the oscillator tube 50 are applied from the .inductor l53 to the input of the block 58 representing the horizontal output circuit -of the television receiver.

In accordance with conventional practice, the horizontal output circuit 58 includes a horizontal output tube and an output transformer in which is developed the sawtooth currents for the .horizontal deflection coils of the cathode ray tube. These currents are applied on output terminals 59 to the terminals H-H of the horizontal deflecting coils 61of the cathode ray tube 14. The hori` zontal output tube is alternately rendered conductive and nonconductive at each cycle of the oscillations produced byv the oscillator 50 to provide the horizontal deflection currents in accordance with conventional reaction scanning techniques.

The oscillatoryoutput of the tube 50 is also fed back' to t-heV phase detector circuit by means of a coupling capacitor 36 connected from the junction between resistors A56 and 57 to the junction point 34. Also -fed back. Vto

the phase detection circuit are sawtooth pulses produced by the horizontal output circuit 58. These sawtooth..

pulses are connected by conductor 62 through couplingcapacitor 35`to the junction point 34.

In operation, `the phase detector circuit has two inputs,

vnamely the junction point 28 and the junction point 34..

To the junction point 28 are applied the horizontal synchronizing signals from the sync signal separator 19 which may have the shape illustrated in FIGURE 1 by .the waveform 63.'V These represent a series of tirst periodic signals occurring at the horizontal synchronizingscanning rate as established by the transmitted composite signal.

To the junction point 34 are applied lthe outputs from the horizontal oscillator tube'50. and the horizontal output circuit 58. The waveform-applied to the junctionpoint 34 as a series of secondperiodic signals is represented by the waveform 64 in FIGUREfl. Since the horizontal oscillatortube 50 is tuned to oscillate at substantially lthe horizontal scanning frequency both waveforms 63 and 64 occur atsubstantially the same repetition rate or. frequency.

The load for the phase detectorcircuit may be taken `as the capacitor 37, ilter 39, 41, and the input circuit of Vthe horizontal control tube 40.ignoring for the time y.being .the resistor 38. The peak detector circuits reprewill. substantially eliminate.

4 sented respectively by diode 29. =withiits load resistor 32A anddiodei 31lk :with its loadresistor 33appear in parallel to the horizontalfsynchronizing signals.63 appliedto the junction 28.1` Thesegsatuezr peak detector circuits appear in series to the psignal 64 applied to the junction y34. In accordance Withknown balanced phase .detection techniques, a voltageis produced across` the capacitor 37;` which is a function kof the diierence. in phase:` between Athe .i horizontal synchronizing signals applied to junction 28 andthe signals applied .to` junction 34.. Briey, the peak detectors made up of diode 295 and .resistor 32 and diode 31 'and resistor 33 produce y average. voltages across the respective `resistors 32 and.33 which areequal 'approxif mately to the peak of the negative pulses of the horizontal synchronizing Lsignals 63 superimposedY in opposite po.

larities on the horizontal output signals 64,` during retrace time.

If the sawtooth pulses 64 Iappliedto junction 34 .are exactly in phase with the .horizontal synchronizing pulses 63,; the sawtooth voltage. is; passing through its A.C.

yaxis in one. direction, say negative going, jin this-r case, at,

the same instant that .the horizontal synchronizing pulse,

also negative going, is causing :the .diodes 29 and 31r to conduct. Therefore, during such in-phase. conditions there will be'no D.C. voltage -developed across the resistors 32 and 2:3,1i.e.,V the respective voltages across these resistors is Vthereorectically balanced out. If. the `horizontal oscillator isoperating at a lower frequency so that plied tothe horizontal =control tube .toslow down the oscillator. Hence the.. oscillator tends to be stabilized at a synchronous frequency substantially in phase-with Y the-horizontal synchronizing voltage.: Y

yThe conventional balanced phasedetector, as its opt eration'is thus far. described,.may. have several disadvantages, as illustrated by the waveforms in FIGURES 2a and'3a`, which disadvantages the circuit of FIGURE l The :waveforms of FIG- URES 2a and 3x1-are .taken in ther absenceof resistor 38.

In FIGUREZ `and more particularly in subFIGURE 2a,

-theprior art waveform 63 appearing at the Youtput-of sync signal separator 19 is illustrated on a .much longerA time scale than that ofMFIGURE! .1,2 and therefore appears greatly compressed. Two vertical synchronizing intervals A711are-shown..y It can be seen-that, ,after the occurrence .offeach vverticale synchronizing interval, a

ringing clectzmay be present evidencednby the damped l oscillation Iindicated at` 72 in the :envelope of the waveform 2a.- Due to such ringing ,the` phase detection'cir-z` cuit may iwander-` or ;hunt about the proper synchro-y nizing ttrequency for the :horizontal oscillator. 'I'his may cause the beginning` of the displayed=videorsignal to. also hunt, ,possibly producing 1a bendingy ofl the ltop portion of the reproduced image.

A'second possible disadvantage. of priorart circuits is showninFIGURESoz.y In this vligure the waveform appearingacross the Iload ,capacitor 37 is illustrated on Va timescale comparable to that of .FIGURE 2. i The averagevalue of thewaveform 65is appliedrto the control electrode ofthe horizontal controltube .401for correction of any phase diierences between thecutput of the horizontal oscillator andthe incoming horizontal synchro#v nizing pulses.A The tdips `73 .in the `waveforrnS.y of FIGf URE 3a show'V that there is a corresponding change-orl dip gin Vthevaverage,` value yof the .signal which occursV on those occasions when the horizontal sync of the television .receiveris momentarily lost. This may loccur when the receiver is switched from channel to channel or during the vertical retrace or due to interference, etc. This large change in the signal which is applied to the horizontal control tube may `again contribute to hunting or loss of information in the .portion of the reproduced image occurring during or immediately -a-fter such time interval.

Now consider the effect of adding the resistor 38 between t'he junction point 34 and ground. The resistor 38 serves as a ground return for the anode of the diode 29. In the absence of the resistor 38, the diode 31 which has a grounded anode has a much better response to low frequency components in the horizontal synchronizing signal than the diode 29. This low frequency response may cause an error signal to -be applied to the horizontal control tube even though the horizontal oscillator may in fact be properly synchronized with respect to the incoming horizontal synchroninzing signals.

The low frequency components which may be troublesome are generally present in the horizontal synchronizing signals occurring during the vertical sync and vertical retrace interval. By adding the ground return resistor 38 for the lanode of diode 29 both of the diodes respond to the low frequency components more nearly in substantially `the same manner so that these low frequency components tend to be balanced out. This balancing reduces `any error signal that may be present due to the differences in the frequency response of the diodes k29 and 31.

- 2a and 3a, except that resistor 38 has now been added to the circuit, are shown in FIGURES 2b 'and 3b, respectively. From FIGURE 2b it can be seen that the presence of the ground return resistor 38 in the circuit substantially eliminates the ringing from portion 74 of the signal occurring immediately after the vertical blanking interval thus reducing the undesirable hunting that may otherwise be present immediately following the vertical sync interval.

Similarly, with the ground return resistor 38 present in the circuit, FIGURE 3b shows that changes in voltage due to loss of sync or other interference are reduced by the wave shaping effect of the resistor 38. The dips 76, it can be seen, are substantially smaller than the dips 73 of FIGURE 3a. Thus, utilization of the resistor 38 provides quicker recovery of the horizontal oscill-ator after the loss of sync that may occur for various reasons.

Thus, the output waveform of FIGURE 3b shows that the phase of the oscillator 50 signals now follows more closely the phase of the horizontal synchronizing signals than without the resistor 38. The voltage p-roduced across capacitor 37 is Iapplied to the control electrode of the horizontal control tube 40 by way of the filter networks made up respectively of resistor 39 and capacitor 41 and resistor 42 and capacitor 43.

The tube 40 effectively acts as a D.C. amplifier, its output being applied to the control electrode of the oscillator tube 50 by way of horizontal hold potentiometer 49 and resistor 52. The frequency of oscillator 50 is thereby tuned by this amplified voltage to exactly the proper synchronized frequency. The output signals from tube 50 `are applied to the horizontal output circut represented by the block 58 for the production of horizontal deflection waveforms, as explained above.

Without any intentto limit the scope of the present invention a circuit utilizing the following components has been found to have the beneficial effect represented by the waveforms 2b and 3b:

Capacitor 27-47 Diodes 29, 31--GE Dual Selenium Capacitor 30-68 Resistors 32, 32a- 390,000 ohms each Capacitor 36-27 Capacitor 37-820 5 Resistor 38-120,000 ohms Resistor 39--270,000 ohms As has been explained above, the improved results obtained by the circuit illustrated in FIGURE l have been attributed to the improved response of the diode 29 to low frequency components that may be present in the horizontal synchronizing signal. It is possible that the same results may be achieved by otherwise equalizing the respective frequency responses of each of the diodes. It might be possible for example to balance the respective low frequency responses of diodes 31 and 29 by inserting an impedance between the anode of diode 31 and ground. Since the detector is preferably solely phase sensitive, any modification which balances the responses of each of the respective diodes is desirable.

While the above invention has been described with respect to improvements in an automatic frequency control circuit for use in synchronizing av horizontal oscillator in a television receiver, it is clear that the same principles may be applied to other frequency control or phase detection circuits to achieve the benefits above described.

What is claimed is:

1. An automatic frequency control comprising, in cornbination,

a first and a second peak detector circuit;

a source of first periodic signals occurring at a first frequency, said first periodic signals being applied to said first and second peak detector circuits arranged in parallel;

a source of second periodicfsignals occurring at substantially said first frequency, said second periodic signals being applied to said peak detector circuits connected in series opposition, a capacitive load circuit, said first and second peak detector circuits producing a control voltage across said load circuit when said first and second periodic signals are applied thereto, said control voltage being proportional to any phase difference between said first and second v periodic signals;

control means responsive to said control voltage and connected to said source of second periodic signals for changing the frequency of said periodic signals in a direction to reduce said control voltage to substantially zero; and

impedance means coupled across said load circuit for substantially reducing the response of said first peak detector circuit to low frequency variations of the periodic signals supplied by said first source thereby tending to equalize the frequency response of said peak detector circuits.

2. An automatic frequency control arrangement for television receivers comprising, in combination,

a first peak detector circuit having a first parallel combination of adiode and a resistor;

a second peak detector circuit having a second parallel combination of a diode and a resistor, the anode of said diode of said second combination being arranged substantially at electrical ground potential;

a source of horizontal synchronizing pulses occurring at the horizontal synchronizing frequency of the television receiver, said signals being applied to said rst and second peak detector circuits arranged in parallel;

a source of second periodic signals occurring substantially at the horizontal synchronizing frequency of the television receiver, said second periodic signals being applied to said first and second peak detector circuits arranged in series;

a capacitive load circuit coupled across the series combination of said first and second peak detector circuits;

said peak detector circuits thereby producing a control voltage across said load circuit varying as a function of the phase difference between said horizontal synchronizing pulses and said second periodic sig'- nals;

control means responsive to said control voltage forl changing the frequency ofsaid second periodic signals in a direction to reduce said phase difference substantially to zero; and

impedance means coupled between the anodeof the diode of said first parallel combination and ground potential across said capacitive loadcircuit for substantially reducing the response of saidV first peak tude of said horizontal synchronizing pulses thereby tending to equalize'the frequency response of said peak detector circuits. 3. An automatic frequency control arrangement for television receivers comprising, lin combination,

a first peak detector circuit having a first parallel cornbination of a diode and a resistor;

`10 detector :to low frequency variations of the amplif a second peak detector circuit having a second parallel 1 combination of a diode and a resistor, the anode of saidl diode of said second combination being z arranged substantially at electrical ground potential;

a sourceV of horizontal synchronizing pulses occurring at the horizontal synchronizing frequency of the television receiver, said signals being applied to said first and second peak detector circuits in parallel; a source of second periodic signals occurring substantially at the horizontal synchronizing frequency of the television receiver, said second periodic signalsg..

being applied to said first and second peak detector.V

circuits in series;

a capacitive load circuit coupled across the series comf bination of said first and second peak detector circuits;

said peak detector circuits thereby producing a control voltage across said load circuit varying as a function of the phase difference between said horizontal syn'YA chronizing pulses and said second periodic signals; control means responsive to said control voltagefor changing the frequency of said second periodic sig. nals in a direction .to reduce said phase difference;

and

impedance means includingrat least one resistor for returning the anode of the diode of said first parallel combination to ground potential for substantially reducing the response of said first peak detector to.

low frequency variations of the amplitude of said horizontal synchronizing pulses thereby tending to equalize the .frequency response of said peak detector circuits.

4. An automatic frequency control arrangement for television receivers comprising, in combination,

8E the television receiver, said secondtiperiodic signals being applied to saidfirst and. second peak detectorv circuits in series;-

a capacitive load circuit coupled .across the. series combination of said first and second peak detector circuits;

said peak detector circuits thereby producing a control voltage acrosssaid load circuit varying as a function of thephase dierence Vbetween said horizontal synchronizing .pulses and 'said second periodicsignals;

control means responsive `to said vcontrol voltage for changing the frequency of saidv secondfperiodic signails. in a clirc'ectionrtoreduce said phase difference; an

impedance'means including a compensatingy resistor for returning the anode of the diode of said first parallel combination 'toV ground zpotential,"the value of resistance of said compensating resistor-being substantially smallerl than the value ,of resistance of. either of said resistors in said first and second parallel cornbinations toireduce substantially the low frequencyl response of said first peak detector without'substantially affecting the low frequency response. .of said second peak detector.

5. An automatic frequency .control arrangement yfor television receivers comprising rectifierl to low frequency variations in amplitude of the periodic signals. supplied vby said first source thereby tending to.4 equalize the .frequency response of said rectifiers. 6: An automatic: frequency control arrangement for television receivers according to claimS Wherein the` parallel combination of said first :rectifier and said first resistor is connected inseriesrelation with said capacitive load circuit {across said first sourcel and the parallel combination of said second rectifier and said second resistor=is connecteddirectly acrosssaid first source.

ReferencesCitedy the Examiner a UNITED .STATES PATENTS FOREIGN PATENTS 1,255,296 1/1961 France.

DAVID G.Y REDINBAUGH, Primary' Examiner..

J.. MCHUGH, Assistant-Examiner. 

1. AN AUTOMATIC FREQUENCY CONTROL COMPRISING, IN COMBINATION, A FIRST AND A SECOND PEAK DETECTOR CIRCUIT; A SOURCE OF FIRST PERIODIC SIGNALS OCCURRING AT A FIRST FREQUENCY, SAID FIRST PERIODIC SIGNALS BEING APPLIED TO SAID FIRST AND SECOND PEAK DETECTOR CIRCUITS ARRANGED IN PARALLEL; A SOURCE OF SECOND PERIODIC SIGNALS OCCURRING AT SUBSTANTIALLY SAID FIRST FREQUENCY, SAID SECOND PERIODIC SIGNALS BEING APPLIED TO SAID PEAK DETECTOR CIRCUITS CONNECTED IN SERIES OPPOSITION, A CAPACITIVE LOAD CIRCUIT, SAID FIRST AND SECOND PEAK DETECTOR CIRCUITS PRODUCING A CONTROL VOLTAGE ACROSS SAID LOAD CIRCUIT WHEN SAID FIRST AND SECOND PERIODIC SIGNALS ARE APPLIED THERETO, SAID CONTROL VOLTAGE BEING PROPORTIONAL TO ANY PHASE DIFFERENCE BETWEEN SAID FIRST AND SECOND PERIODIC SIGNALS; CONTROL MEANS RESPONSIVE TO SAID CONTROL VOLTAGE AND CONNECTED TO SAID SOURCE OF SECOND PERIODIC SIGNALS FOR CHANGING THE FREQUENCY OF SAID PERIODIC SIGNALS IN A DIRECTION TO REDUCE SAID CONTROL VOLTAGE TO SUBSTANTIALLY ZERO; AND IMPEDANCE MEANS COUPLED ACROSS SAID LOAD CIRCUIT FOR SUBSTANTIALLY REDUCING THE RESPONSE OF SAID FIRST PEAK DETECTOR CIRCUIT TO LOW FREQUENCY VARIATIONS OF THE PERIODIC SIGNALS SUPPLIED BY SAID FIRST SOURCE THEREBY TENDING TO EQUALIZE THE FREQUENCY RESPONSE OF SAID PEAK DETECTOR CIRCUITS. 